Although an "ICAO" number, 1:50 should not be cast in stone, either...
Just to clarify, ICAO doesn't actually specify a particular ratio. Chapter 12 of Annex 6 to the Convention on International Civil Aviation relates to the requirements for Cabin Crew. Section 12.1 addresses the Standard for the assignment of emergency duties as follows:
An operator shall establish, to the satisfaction of the State of the Operator, the minimum number of cabin crew required for each type of aeroplane, based on seating capacity or the number of passengers carried, in order to effect a safe and expeditious evacuation of the aeroplane, and the necessary functions to be performed in an emergency or a situation requiring emergency evacuation. The operator shall assign these functions for each type of aeroplane.
Most States have chosen a ratio based on the seating capacity (1 cc per 50 seats). There are a few that have selected 1 cc per 50 passengers.
The most conservative right now remain Australia at 1:36 passengers and Canada at 1:40 passengers. However, even here in Canada, the DH83, CRJ and ATR42 can all operate at 1:50 seats and there is a proposal to provide an option to all carriers to be able to use a 1:50 seats ratio.
It's interesting that this proposal in on the table in Australia again ... I recall that earlier this decade it had been turned down.